In modern information processing equipment, high-speed printers in which the characters are printed without impact of printing type in relief on a receiving sheet of paper are increasingly being used. Magnetographic printers of this generic type similar to those described and shown in French Pat. No. 2.305.764 are already known. In such printers the printing of the characters is formed by first forming a latent magnetic image on the surface of a magnetic recording carrier generally having the shape of a rotating drum or an endless belt, the image being derived from signals received from a control unit. In a known manner, this latent image comprises a set of magnetized zones of very small dimension, virtually punctuate, which are conventionally known as magnetized points.
The latent image thus formed is then developed, or in other words made visible with the aid of a powdered developer, which comprises particles of thermoplastic resin enclosing each of the magnetic particles and so is not attracted except by the magnetized zones of the recording carrier, thus forming an image in powder on the surface of this carrier. After that, this powder image is transferred to the receiving sheet of paper. In order to obtain high-quality printing of the printed characters on the sheet of paper, latent magnetic images of which the various constituent points are very small and very close to one another must be recorded on the magnetic film of the recording carrier. To record these latent images, a recording device known as a transducer is used, which includes a plurality of magnetic heads arranged one beside the other. Each of these heads is formed by a metal core of high magnetic permeability on which a winding is wound; at one of its ends this core has a magnetic pole, known as the writing or recording pole, which is placed in contact with or near the magnetic film such that each time an electric current of suitable intensity is sent through this winding, the current creates a magnetic flux in the core that magnetizes the portion of the magnetic flux located facing the writing pole of the head.
The magnetized points thus formed by the various heads of the transducer must not only be perfectly defined and all have substantially the same size, but they must also have virtually the same intensity of magnetization, in order that each can retain a quantity of developer particles that remains virtually constant from one point to another. To meet these requirements, it is indispensible that the distance that separates each head from the magnetic film remain constant and less than a predetermined limit value. This is why, for example, in the case where the magnetized points formed on the magnetized film each have a square section on the order of about 100 .mu.m on a side and are distributed at a density such that the distance separating two adjacent magnetized points is practically equal to 30 .mu.m, this limit value has been found to equal about 10 .mu.m. Adjusting the heads and the recording carrier to keep this distance as constant as possible has always been a difficult problem to solve and could not be surmounted except by using complex and particularly expensive equipment. For this reason, to avoid having to use such equipment, it has in practice been necessary to put the transducer heads into contact with the magnetic surface of the recording carrier. However, putting the heads into contact with the magnetic surface causes heating upon displacement of the recording carrier, and this effect is all the more pronounced, the higher the speed with which this carrier is driven.
This heating is capable of interfering with or even preventing recording of the information on the carrier. Moreover, it causes the developer particles deposited on the carrier to soften, so that they cannot be completely transferred to the sheet of paper. Over the long term, a film of resin that increases in thickness with time forms on the magnetic film of the carrier, and this film causes the distance between the recording heads and the magnetic film of the carrier, and this film causes the distance between the recording heads and the magnetic film to increase constantly, thus rendering the printing of the characters completely defective.
In order to diminish friction of the heads on the recording carrier and thus to notably reduce the heating caused by this friction, one might think to deposit solid particles of lubricating substance on the magnetic film of the recording carrier, these particles being such as have already been used in the prior art to assure lubrication of the recording surface of electrostatic printing machines. For example, electrostatic printers are known which have been described in Japanese published patent application No. 56.101177, in which the surface of the electrostatic drum is lubricated by applying a strip of fabric, impregnated with solid polyethylene, polytetrafluoroethylene or paraffin wax to this surface.
An electrophotographic copying machine is also known, described in Japanese published patent application No. 57.078578, in which the photoreceptive electrostatic surface is lubricated by depositing on this surface a solid lubricant comprising polyethylene or zinc stearate particles, which is done either directly or by putting a strip of fabric containing these particles into contact with the surface, or by incorporating these lubricant particles with the developer particles intended to be applied to the surface.
However, it has been observed that by depositing particles of lubricating substances already used for lubricating an electrostatic printer recording carrier on the magnetic film of the recording carrier of a magnetic printer, these lubricating particles became charged with static electricity because of the friction they underwent upon passing beneath the transducer, and they were then capable upon passing before the applicator device of the developer of attracting resinous developer particles contained in this device. As a result the developer articles, which were also attracted by the lubricating particles, formed an undesirable film when they were then transferred to the sheet of paper, and this film greatly reduced constrast between the image intended to be printed and the original base of the paper.